The performance demands being placed on optical components are increasing rapidly with rapid changes in optical technology. This situation is driving rapid changes in methods for testing the ability of optical components to withstand the rigors of modern optical applications. Such optical components include, but are not limited to, any of various optical elements used in optical systems or used singly for optical purposes, optical elements having an antireflective or other coating, and transmissive and reflective optical elements including photomasks and substrates for use in microlithography.
Particular concern is directed to the long-term stability of optical performance of optical components exposed to extremely long or extremely short wavelengths of light compared to visible light. For example, very short wavelengths (&lt;200 nm) of light generated by any of the various types of excimer lasers are being increasingly employed in optical systems in which the short, high-intensity light is useful for producing improved image resolution. Such systems include laser-processing devices, and microlithography apparatus such as steppers.
In any event, such extreme wavelengths of light (compared to visible light) place severe demands on optical components. This poses a widespread need for methods and systems for measuring and evaluating damage to, or other undesirable change in performance of, optical components from exposure to such wavelengths.
Photons of short-wavelength light have higher energy than photons of visible light and are generally more likely to interact with an object on which such light impinges. Such interaction can result in a progressive degradation of an optical property of the object, such as an increase in light absorption and an accompanying decrease in light transmission through the object. Degradation in an optical property can have enormous impact on the suitability of the optical component for a particular use. Therefore, it is very important to be able to measure and evaluate such degradation as part of the engineering effort required to design an optical system.
Many optical components are surficially coated to reduce reflections, for example. Hence, a degradation of optical performance of an optical component may be caused by damage or other undesirable change to the coating rather than, or in addition to, damage to the component itself. In addition, optical components used in a particular atmosphere or other environmental condition may exhibit a performance degradation due to light-induced changes to residual lens-polishing material or other substance adhering to or absorbed by the optical component.
Performance degradation due to a change in a physical property of an optical component is typically progressive and can be due to factors such as radiation-induced heating (i.e., heating caused by irradiation) and damaging effects of the electric-field component of high-intensity short-wavelength light irradiated onto the optical component. Hence, it is important to be able to evaluate changes in optical performance of an optical component over time or otherwise with cumulative exposure to light. Conventional methods for performing such evaluations include destructive test methods such as laser-damage threshold (LDT) testing.
As noted above, degradation in an optical property (such as a reduction in transmittance or reflectivity) can arise from surficial adhesion and/or absorption of a contaminating material from the environment, a phenomenon termed "fogging" or "clouding". Fogging can arise not only in optical elements comprising an optical system (e.g., projection-optical system) but also in any of various materials used in conjunction with the optical elements (e.g., lens mountings, antireflective coatings, or photoresist films used in microlithography).
Fogging can arise simply from placing an optical component in environmental contact with a culprit contaminant (capable of adhering to or absorbing into the optical component). In other cases, whether or not the contaminant adheres to or is absorbed by the optical component depends upon whether the optical component and/or contaminant is irradiated by light. The latter is generally understood to result from heating of the optical component or contaminant by exposure to the light, resulting in a photochemical reaction leading to fogging (e.g., by photochemically induced generation of a gas from the contaminant, wherein gas condenses or precipitates on the optical component). This problem is particularly acute in optical apparatus that use short-wavelength light which seems to aggravate the fogging phenomenon. Furthermore, optical components in such apparatus can be exposed to scattered light (as opposed to direct light) which can also lead to fogging.
Satisfactory methods and apparatus do not exist for evaluating fogging from adhesion and/or absorption of substances (e.g., organic substances) to optical components and the effect of such fogging on light absorption by the optical components, especially such phenomena not accompanied by visual damage to the components. Even slight fogging or other degradation that is not visually detectable can have a substantial effect on the performance of an optical component. By the time physically observable degradation is manifest, optical performance may have become profoundly reduced.
Conventional methods for evaluating the effects of light intensity on a sample employ light-intensity sensors. However, such methods have practical limits. For example, it is difficult to control and measure the stability and light output of a short-wavelength (e.g., 200 nm or less) light source. The resulting variability in obtained measurements makes it especially difficult to reliably measure small changes in light absorption by a sample optical component.
Other problems with conventional approaches include: (1) Whenever the subject light is being produced by a pulsed laser (e.g., excimer laser), the response of the light sensor is usually delayed relative to each received pulse. (2) It is difficult to separately measure a unit of light absorption by conventional optical methods because, inter alia, such measurements include the effect of light scattering from the surface irradiated by the light. (3) Conventional methods do not facilitate ascertaining the origin and effect of contaminant material adhering to and/or absorbed into the surface of the sample optical component as a result of thermal or photochemical reactions caused by light irradiation of a source of the contaminant material. (4) Although conventional methods include techniques of "optical cleaning" (by which optical components are removed from their mountings and any adhering matter is cleaned from the surfaces of the components using light), the optical components tend to become readily recontaminated after such cleaning.
Finally, although certain conventional testing methods involve exposing a sample optical component to unusually intense light so as to accelerate the effect of light exposure, there is a significant probability that the sample under such conditions will exhibit a change in light absorption due to color shifting and similar phenomena arising from the intensity of the light, and not due solely to cumulative exposure to the light. This problem is especially prevalent when the surface of the sample optical component is coated or is contaminated with adhering or absorbed matter. Under such conditions, if the light absorbed by the optical component itself exhibits a variation, such a variation is impossible to separate from a change in light absorption exhibited by the coating and/or a change in light absorption exhibited by a surficial contaminant. Furthermore, with respect to optical components coated with a thin film, accurate measurements of changes in absorption of the optical component are not obtainable because changes in light absorption also typically accompany degradation of the thin film with exposure to the same light.
A conventional attempt to solve this problem involves irradiating the component with light pulses and measuring, at various depths within the component, acoustic waves generated in the optical component due to exposure to the light pulses. The frequency of the light pulses is varied while changing the thermal diffusion conditions in an attempt to obtain light-absorption measurements at various depths within the sample optical component. However, when attempting to obtain measurements at extremely short wavelengths of light, it is conventionally not possible to irradiate using an intense light source while changing the frequency of the light. Consequently, this technique cannot be used reliably to quantitatively separate and evaluate changes in light absorption on the surface and changes in light absorption through the thickness dimension of the optical component.